Heretofore, various materials have been used for guides for movable systems for magnetic tapes such as VTR and cassette tapes. For instance, known organic materials involve polyoxymethylene (POM for short) which are used alone or in combination with a fluororesin and even silicon. Some plastic parts contain carbon so as to release charges generated by frictional contact with the surface of a tape, since plastics tend to be electrified. Known inorganic materials include metals and ceramics, while known metallic guides are formed of stainless materials or brass plated on its surface with nickel or chromium.
Such guides as mentioned above operate in two manners, according to one of which they rotate in association with the travelling of a tape. According to another, they are fixed relative to the travelling of a tape. In most cases, the fixed type of metallic guides are used for portions through which a tape travels under relatively increased tape tension with increased changes in the direction of its movement.
Some problems with such conventional guides originate from the properties of the materials used and some arise from the processing techniques applied.
Of noteworthy in connection with the materials used is that in view of the principles of friction or tribology, a material of larger momentum should basically be harder than its associated material of smaller momentum. In other words, a tape's material should fundamentally be harder than its associated guide. Under such conditions, the amount of abrasion is considered to be minimized.
In view of tribology, however, the material used for the conventional guides is totally reversed in combination or not in ideal conditions at all. More specifically, the POM that is a polymeric material used for guides is much harder than PET that is a base film of magnetic tapes. In addition, stainless or chromium-plated brass materials used for fixed guides are harder than the POM. This implies that if guides become irregular even slightly on their surfaces, then magnetic tapes will be likely to be damaged.
To avoid this problem, the surfaces of guides over which tapes travel, i.e., in contact with them, are now polished into mirror surfaces to increase the area of their surfaces in contact with the tapes, thereby applying local contact pressure to the tapes for the purpose of preventing the travelling resistance from increasing and so the surfaces of the tapes from being damaged. More illustratively, the guides are polished to such a surface roughness as expressed in terms of Rmax equal to or less than 0.1 .mu.m.
For the rotary guides, on the other hand, a roundness of 0.1 .mu.m or below is required in addition to having such a surface roughness as mentioned.
Thus, a problem with the conventional guides is that as a result of unavoidably making no account of the basic principles of tribology in view of some limitations placed on materials, it is required to increase their accuracy of processing to the highest level now achievable, leading to increases in product costs. In the case of the fixed type of guides in particular, it is still impossible to make them of plastics. Thus, metallic guides are still forcibly used even for cassette cases or decks, imposing considerable limitations on making them light, mass-producing them and cutting down their costs.